This is a reasonable build, but discussion of item choices (and alternatives) is missing. Usually builds are broken down along in order of purchase, so that Philosopher's Stone and Heart of Gold would be listed separately. Furthermore, I'd get Heart of Gold and Kindlegem before ever finishing Randuin's Omen, for example. Health and CDR are just that good on this champ.

You clearly know how to play Leona, but your guide itself is lacking some necessary content and coding.

One thing I would definitely add is a teamfight section. Things such as primary targets, timing on your ult, and defending the carry would be good things to discuss and explain in this section. Additionally, you don't explain your mastery choices. While they may seem obvious to you, some people might not know what's so great about Greed . Lastly, I'd atleast put in even a short explanation to warding, since an important job of supports is to ward.

Next, you have no pictures in this guide. You can use the code:
[ [Leona]] *Remove the space to get Leona
[ icon=leona size=40] *Remove the space to get

Besides that, your English is understandable, and your build is very good :D
I hope to see your guide get even better.